1. Field of the Invention
The invention relates to a hydrogenation process in which different boiling fractions of a coal-derived solvent are subjected to hydrogenating conditions that maximize the concentration of efficient hydrogen-donating hydroaromatics.
There is a continued recognition of the need to improve the hydrogen donating ability of a solvent in a coal liquefaction process. This objective has been sought after by many researchers but a satisfactory solution has yet to be found.
For a long time coal chemistry research has focused on providing more effective hydrogen-donating species for use in coal liquefaction processes. These molecules have the ability to donate hydrogen to liquids being made from coal at elevated temperatures in liquefaction reactors. Liquids are made from the coal when the chemical bonds in a coal molecule are thermally degraded. In the presence of these more effective hydroaromatic species, described in more detail below, the coal conversion in the coal liquefaction reactor is increased as is the production of the more desirable heptane soluble oils (liquids).
In general a coal liquefaction process consists of heating a slurry of coal and some solvent at temperatures of about 700.degree. to 880.degree. F. Heating may occur under a hydrogen atmosphere and at pressures of about 0-4500 psig, usually about 800-3000 psig. Under these conditions the coal is thought to undergo a thermally-induced free radical fragmentation. Coal conversion yields are related to the efficiency with which these free radicals are stabilized. The preferred mode of radical stabilization has been found to occur by hydrogen abstraction from hydroaromatic species that are present in the solvent. Molecular hydrogen is also present during liquefaction reaction in an attempt to replenish the hydrogen consumed from the hydroaromatic species, to stabilize any remaining radicals directly, and to help prevent coking.
A method for treating a coal-derived solvent has now been discovered in which the concentration of efficient hydrogen-donating hydroaromatics is increased. In contradistinction, the prior art hydrogenating processes have, for the most part, sought to maximize without selectivity the concentrations of all hydroaromatics and not just those that have now been found to be more effective hydrogen donors. It was heretofore incorrectly assumed in the prior art that the effectiveness of the solvent was related to the total hydroaromatic content of the solvent.
It has now been found that not all hydroaromatics are equally as effective in donating hydrogen during the coal liquefaction process. In fact, it has been found that tetralin--a hydroaromatic whose presence was sought to be increased in prior hydrogenating methods and which is considered by the art as a representative hydrogen-donating hydroaromatic--is not as effective in donating hydrogen as, for example, tetrahydrofluoranthene and dihydrophenanthrene: these two hydroaromatics were found to be 1130% and 810%, respectively, more reactive than tetralin in the conducted experiment which simulated the reaction conditions that take place during a coal liquefaction reaction.
The known prior art processes tend to be inefficient in that they serve to increase the concentration of certain hydroaromatic compounds which have been found to be less effective hydrogen donors. Although the prior art recognized that, as a class, all hydroaromatics were much better hydrogen donors than either saturates or aromatics, it was not recognized that certain hydroaromatics were surprisingly superior as hydrogen donors in the specific coal liquefaction process. The prior art liquefaction hydrogenating processes are not selective; whereas, the instant invention is highly selective serving to increase the concentration of more highly effective hydrogen-donating hydroaromatics such as tetrahydrofluoranthene and dihydrophenanthrene.
2. Description of the Prior Art
U.S. Pat. No. 4,303,498 discloses in its broadest aspects subjecting the different boiling fractions of a solvent for coal liquefaction to one or more hydrotreating processes. It was found that the fraction boiling at more than 485.degree. F. possessed virtually no hydrogen-donating properties, but the fraction boiling at less than 485.degree. F. did comprise hydrogen-donating substances when hydrogenated. It was accordingly subjected to hydrogenating treatment, with the fraction boiling from 390.degree.-415.degree. F. preferably hydrogenated once, and the fraction boiling from 415.degree.-485.degree. F. preferably hydrogenated twice. Hydrogenation conditions were the same for each boiling fraction; a reaction temperature of 750.degree. F., a reactor time of 60 minutes, and an initial hydrogen pressure of 70 kg/cm.sup.2 G. Thereafter, the fractions boiling at below and above 415.degree. F. were recombined in certain set proportions. By subjecting the fraction boiling at 415.degree.-485.degree. F. to two hydrogenating treatments, the heavy liquid was first dealkylated and subsequently hydrogenated--that is, alkylnaphthalene and other components of the heavy liquid were dealkylated to form naphthalene and subsequently hydrogenated into tetralin.
U.S. Pat. No. 3,726,785 discloses liquefying a first and second slurry of a particular coal employing two separately obtained coal-derived solvents. A coal-derived solvent is separated into what is called a high and a low boiling solvent fraction at a temperature cutoff of preferably about 500.degree.-600.degree. F. Each fraction is then separately hydrogenated in a hydrogenation reactor at a temperature within the range from about 650.degree.-850.degree. F., preferably about 700.degree. F., and pressures in the range from about 650 psig to about 2000 psig, preferably about 1300 psig. With each of these solvents a separate slurry of coal is formed and, in turn, the slurries are separately liquefied.
U.S. Pat. No. 4,323,447 discloses a coal liquefaction process in which the coal liquefaction solvent containing tetrahydrophenanthrene is subjected to downstream catalytic hydrogenation to convert a portion of the tetrahydrophenanthrene (THP) present therein to octahydraphenanthrene (OHP). The hydrogenation reaction is conducted under temperatures between about 644.degree. and 725.degree. F. and with hydrogen partial pressures in the range of about 1000 to 2500 psi and a liquid hourly space velocity (LHSV) of between about 0.2 and about 10 hr.sup.-1.
U.S. Pat. No. 4,311,578 discloses a method wherein a solvent or diluent employed in a liquefaction of solid carbonaceous material comprises from about 65 to 85 wt % of hydroaromatic components. A solvent fraction separated out from the liquefaction product is subjected to hydrogenation and the naphthenic components contained therein are extracted. These extracted naphthenic components are then dehydrogenated and subsequently hydrotreated to produce additional hydroaromatic compounds that are then returned to the solvent from which they were extracted.
Paper 62b entitled "Coal Solvolysis in a Series of Model Compound Systems" presented on Nov. 18, 1982 at the AICHE 1982 Annual Meeting discloses that the hydrogen donor content of a solvent alone would not be sufficient to completely define its effectiveness as a coal liquefaction solvent, that is, different donors have different abilities to liquefy coal.